I really couldn't afford to buy this lens, but I bought it anyway after speaking to some professional photographers about it. I could have got the non-IS, I could have got the F/4 to save money. However, I reckon just get the one you are gonna be eternally happy with - and that is THIS lens. It is a beautiful piece of equipment that endlessly churns out one beautiful image after another.

I shoot everthing from portraits to weddings to landscapes with this thing. I love it. The only other lenses I own are the 16-35L MkII and the 50mm F/1.8.

I honestly can't understand how people can whine and complain about issues such as weight. What is the aim of the game? For me it's about getting the right optics to take beautiful photos. I don't care if I have to go to the gym to work out in order to get the muscles necessary to do the job. If that's what I really had to do, then it would be well worth it.

This lens is essential to anyone who own an EOS camera with more than one lens

This is my first professional grade lens (not including the 50mm F/1.8). I had it since the MkII first came out. I used this site to help make my decision. I figured that while I still had an APS-C at the time, I would upgrade to a full-frame camera to take advantage of it. In the mean time, it has endlessly produced superb results for me in my landscape and modeling photography. Being able to use it in low light has proved invaluable as well - maintaining low ISOs while shooting clothing catalogues in overcast conditions in the forest.
I never realised just how much a pleasure this lens was to use until I played around with other non-L lenses.
I have a wedding photographer friend who has the 17-40 for his 5D and he always borrows my 16-35 when we shoot together.
If you are having trouble deciding whether or not to spend the extra money to get this lens, I would advise you to go ahead if you can afford it. It has given me years of reliable service despite the abuse I have thrown at it (landscape photography with lots of ocean spray and bad weather).
I didin't realise how important it was to have a tough/durable lens when I first started. I am constantly amazed at what this lens puts up with, and I still keep dishing up the punishment. Remember to always keep a filter snugly mounted on it, and it will serve you for a long time with beautiful, crisp, vibrant, bright photos.